In a hammer mill, what is the dominant mechanism of size reduction?\nConsider the interactions among hammers, housing (liner), and the feed particles.

Introduction / Context:
Hammer mills are widely used for coarse-to-intermediate grinding of minerals, biomass, and chemicals. Understanding the breakage mechanism helps optimize rotor speed, screen size, and liner design to balance capacity and product fineness.

Given Data / Assumptions:

• Typical high-speed rotor with pivoted hammers.
• Perforated screens or grates control top size.
• Feed is friable to moderately hard.

Concept / Approach:
Hammer mills impart high-velocity impacts as feed contacts rapidly moving hammer tips and the breaker plates/liners. Secondary grinding occurs by attrition and inter-particle collisions as fragments circulate and decelerate against the housing and screen surface.

Step-by-Step Solution:

Verification / Alternative check:
Design correlations for specific energy and tip speed (V_tip = π D N) assume impact dominance, while wear patterns on liners confirm attrition and rubbing contributions.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming only one mechanism; overlooking the role of screen clearance and recirculation in attrition.

Final Answer:
Both (a) and (b)